Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device having a bottom emission structure.
Discussion of the Related Art
A flat panel display (FPD) device is applied to various electronic devices such as portable phones, tablet personal computers (PCs), notebook computers, monitors, etc. Examples of the FPD device include liquid crystal display (LCD) devices, plasma display panel (PDP) devices, organic light emitting display devices, etc. Recently, electrophoretic display (EPD) devices are being widely used as one type of the FPD device.
Among the display devices, the organic light emitting display devices use a self-emitting element, and thus have a fast response time, high emission efficiency, high luminance, and a broad viewing angle.
FIG. 1 is an exemplary diagram for describing a light output manner of a related art organic light emitting display device, and illustrates an organic light emitting display device having a bottom emission structure in which light is output to a lower substrate.
The organic light emitting display device may be configured in a top emission type where an organic light emitting diode (OLED) is formed on the lower substrate, and light emitted from the OLED is output to the outside through an upper substrate. However, as illustrated in FIG. 1, the organic light emitting display device may be configured in a bottom emission type where the OLED is formed on the lower substrate, and the light emitted from the OLED is output to the lower substrate.
In the organic light emitting display device having the bottom emission type, as illustrated in FIG. 1, an anode, an organic emission layer, and a cathode are formed on a transparent substrate, each of a plurality of pixels is divided by a bank, and the OLED emits light with a current which is transferred by a driving thin film transistor (TFT).
FIG. 2 is an exemplary diagram schematically illustrating a cross-sectional structure of an OLED applied to a related art organic light emitting display device, and FIG. 3 is a graph showing a viewing angle characteristic of the related art organic light emitting display device.
The related art organic light emitting display device having a bottom emission structure, as illustrated in FIG. 2(a), includes a plurality of pixels. An OLED 11 is formed in each of the plurality of pixels.
The OLED 11 may include a plurality of insulating layers such as SiO2, SiNx, and SiOx stacked on a substrate, an anode formed of indium tin oxide (ITO), an organic emission layer which includes a hole injection layer (HIL), a hole transport layer (HTL), an emission material layer (EML), and an electron transport layer (ETL), and a cathode.
As seen in a graph illustrated as a non-cavity in FIG. 3(a), a general OLED has a good luminance viewing angle, and as seen in a graph illustrated as a non-cavity in FIG. 3(b), the general OLED has a good color difference characteristic. However, as illustrated in FIG. 2(a), the general OLED using ITO as an anode has a problem in which it is difficult to secure desired color coordinates by merely changing a general light emitting material. In particular, in the general OLED, it is difficult to secure a good deep blue characteristic.
Therefore, as illustrated in FIG. 2(b), in an OLED applied to another organic light emitting display device, an anode is formed in a three-layer structure including ITO/Ag/ITO. The OLED, having a structure which is as illustrated in FIG. 2(b), uses a micro-cavity. A method using the micro-cavity is disclosed in references such as Korean Patent Publication No. 10-2011-0068638 and Korean Patent Publication No. 10-2011-0064672.
In a related art OLED where an anode is formed in a three-layer structure, due to an effect of a micro-cavity which is formed between aluminum (Al) used as a cathode and the anode having the three-layer structure, an emission spectrum is narrowed, and thus, a more enhanced color characteristic is secured. However, in the related art OLED having the anode having the three-layer structure, as seen in a graph illustrated as a micro-cavity in FIG. 3A, a luminance viewing angle is narrowed, and as seen in a graph illustrated as a micro-cavity in FIG. 3B, a color difference characteristic is degraded. Therefore, is a display device which includes the OLED including the anode having the three-layer structure, it is difficult to secure a viewing angle characteristic suitable for the purpose of using the display device.
To provide an additional description, as shown in FIGS. 3(a) and 3(b), the OLED (illustrated as a micro-cavity) including the anode having the three-layer structure has a front luminance characteristic which is enhanced by 1.8 times in comparison with an OLED including an anode formed of only ITO. However, in the OLED including the anode having the three-layer structure, a luminance viewing angle characteristic and a color difference characteristic are greatly degraded.
In order to adjust the degradation in a luminance viewing angle characteristic and a color difference characteristic, a reflection characteristic and a transmission characteristic of an anode should be adjusted by adjusting a thickness of Al forming a cathode. However, generally, in a manufacturing process, an adjustment range is inevitably limited for securing processability. For this reason, it is not easy to adjust a thickness of Al and a reflection characteristic and a transmission characteristic of an anode in consideration of a viewing angle characteristic.